1. Field of the Invention
This invention relates generally to battery-powered electric drive systems. More particularly, it relates to an improved electric motor which includes a unique stator construction so as to provide variable torque control over a wide range of operating speeds and reduced power supply loading on a battery source, thereby increasing the efficiency of the electric motor.
2. Description of the Prior Art
As is generally known in the art, battery-powered electric drive systems in recent years have become useful in small vehicles such as golf carts, lift-trucks, lawn tractors, and the like. In the prior art battery-powered drive systems, typically a single large electric motor which includes a stator winding has been connected to a source of operating power and a speed control circuit for controlling the operating speed of the motor. The stator winding surrounds a rotor and is inductively coupled thereto so as to rotate a shaft to produce the motor output. In these prior art systems, the power consumption from the battery-powered source will be substantially proportional to the operating speed of the motor. Therefore, in order to produce and maintain a high operating speed of the motor, the electric drive system will require a large power consumption from the limited supply of the battery-powered source. As a result, there is a problem of substantial drainage of the battery charge and thus a very limited useful lifetime for the power source.
This problem is exacerbated for an electric drive system intended for use in an on-road vehicle or electric-powered automobile because it greatly decreases the driving range of the vehicle between battery replacements or rechargings. It is a major obstacle to the development and acceptance of electric-powered on-road vehicles such as passenger cars, trucks or vans, which are desirable to decrease atmospheric pollution and which must achieve acceptance to meet various legal requirements.
A prior art search directed to the subject matter of this application in the U.S. Patent and Trademark Office revealed the following U.S. Pat. Nos.: 009,438 2,026,642 3,422,292 157,299 2,190,223 3,538,364 224,642 2,262,323 4,341,971 233,823 2,471,151 4,352,051 246,010 2,508,172 4,583,014 740,171 2,514,933 5,051,640 869,185 2,564,320 5,063,319 869,186 2,777,104 5,113,100 1,445,933 2,894,157 5,166,568 1,530,449 3,179,825 5,250,861 1,655,286 3,263,147 28,705 1,856,147 3,333,330 T925,001 1,992,725 3,348,111
U.S. Pat. No. 2,564,320 to M. W. Brainard issued on Aug. 14, 1951, discloses a permanent magnet type electric generator in which the circuit to the winding magnetizing the member controlling the magnetic flux includes a variable control, so that the magnetic saturation of the control member may be independently varied.
U.S. Pat. No. 3,263,147 to M. J. Robinett issued on Jul. 26, 1966, discloses a dynamoelectric machine of the commutator type which employs interpoles and includes means for automatically adjusting the strength of the flux at the interpoles in accordance with changes in the load and speed of the machine.
U.S. Pat. No. 3,348,111 to V. H. Woehler issued on Oct. 17, 1967, shows a direct current motor which is energized by a constant potential of direct current. The electric motor has an armature and a plurality of field pole coils, each of which is in series with the armature and in parallel with one another, and has switch means for selectively connecting one or more of the coils directly in series with the armature.
U.S. Pat. No. 4,352,051 to J. H. Johnson issued on Sep. 28, 1982, illustrates a single phase motor which includes a stator assembly having a main winding, a start winding, and an auxiliary winding in which the start and auxiliary windings are connected in two alternative configurations during start and run conditions. In the start condition, the start and auxiliary windings are interconnected by a series connection which causes current to flow through the auxiliary winding in a reverse direction to the flow through the start winding, so that there exists a bucking magnetic relationship between the fields in the start and auxiliary windings. In the run condition, the start and auxiliary windings are interconnected by a second series electrical connection which causes the current to flow through the auxiliary winding in a direction opposite to the first direction, so as to cause the magnetic fields in the start and auxiliary windings to be in an aiding relationship thereby improving motor operating efficiency.
The other patents listed above but not specifically discussed are believed to be only of general interest and to show the state of the art in electric motors having multiple windings. However, none of the prior art uncovered in the search disclosed an improved electric motor having a unique stator design like that of the present invention which provides means for controlling the amount of torque generated over a wide range of operating speeds so as to significantly reduce power consumption but yet to still maintain mechanism speed and performance. This is achieved in the present invention by a unique stator design in which each of the stator poles includes a main winding and at least one fractional-power winding, by which is meant a winding which can provide additional power which is a fraction of the power provided by the associated main winding. A power switching circuit is provided for independently energizing the main and the fractional-power windings on each of the stator poles. A controller is used for selectively controlling the power switching circuit to vary the rotational speed of a rotor and the amounts of torque supplied to a driven member.